1. Field of the Invention
The present invention relates to ultrasonic imaging. More specifically, the present invention relates to a method and apparatus for determining vascular measurements of a subject in an ultrasonic system.
2. Background Information
Doppler ultrasonic imaging has been used in the medical industry to diagnose a wide variety of vascular and cardiac diseases. Due to the nature of these pathologies, vascular measurements over a cardiac cycle are important. One of these measurements is typically the value of the peak velocity of the blood flow during diastole. This measurement is made on a time average spectra of the Doppler signal. If a distribution of velocities of blood is assumed to be Gaussian, then the peak velocity will not be the same as the mean velocity and will have a considerably lower magnitude than the mean or modal velocity. Prior art ultrasound devices typically require operator intervention to mark the peak velocity. This is done usually with a graphic-type cursor that is manually positioned on the visually determined peak on a display of the system. This prior art method has two shortcomings: it is entirely dependent upon the operator, which is subject to error; and it is time consuming.
One prior art automatic method of determining peak velocities is using a fixed magnitude or power threshold to compare power spectral densities and determine the peak velocity. This threshold will be determined empirically for each probe in certain prior art ultrasound imaging apparatus, and specific gain setting used with the probe. Other prior art techniques used a statistical method to estimate a threshold based upon the power of the mean velocity. Both of these prior art methods were neither very accurate nor very robust.
For example, one prior art method of determining peak velocities operated in a manner as illustrated with reference to FIG. 1. Plot 150 of FIG. 1 illustrates a frequency shift (or velocity) versus power spectra of the received ultrasound in a Doppler ultrasonic imaging system. As can be observed, the plot 150 approximates a Gaussian distribution wherein the lowest power the signal is observed near the origin 154, and near Nyquist 155. In this prior art method, it is determined the peak power for the spectra. Then, the system assumes that 90% of the range in magnitude from 0 to the peak 151 exceeds the noise floor of the system. Thus, the velocity that is 90% of the range in magnitude from the peak magnitude is identified to be the peak velocity. This is illustrated as velocity 152 on the spectra 150 of FIG. 1.
Prior art methods, such as the peak velocity detection method above, suffer from the shortcoming that modem Doppler ultrasound systems have better sensitivity and better accuracy than those in the prior art. Thus, new adaptive techniques to determine vascular measurements are required, which exploit the sensitivity of modem ultrasound systems.